Failure protected signal translating system



Dec. 25, 1962 A. F. COOPER 3,0

- FAILURE PROTECTED SIGNAL TRANSLATING SYSTEM Filed March 11, 1960 4Sheets-Sheet l i l I67 I72 I nee I76 n4 I69 I77 OUTPUT 5p 0 INPUT I I750 I760 I660 0 x T l l I I700 I720 FIG. 5

e4 26 INPUT 24 K KI 22 42 INVENTOR.

ARTHUR F. COOPER ATTORNEY Dec. 25, 1962 A. F. COOPER TINPUT 4Sheets-Sheet 2 76 g a 72 B2 74 52 r/ 5 r PICK PICK PICK OFF VALVE =F0FFIr o ll- :4 e2 9O 40 IO FIG. 2

INVENTOR. ARTHUR F. COOPER Qua, 7 ll ATTORNEY Dec. 25, 1962 CQOPER3,070,071

FAILURE PROTECTED SIGNAL TRANSLATING SYSTEM Filed March 11, 1960 4Sheets-Sheet 3 2| I I g m 2 Z 11' I H II I 3 2| 2 Q 09 a 8 F6 Q g 7 K75fi a a U q- N no a J E Q 5 rcu 8 Q 5 8 I33 I I FIG. 3

INPUT INVENTOR. ARTHUR F. COOPER ATTORNEY FAILURE PROTECTED SIGNALTRANSLATING SYSTEM Filed March 11, 1960 4 Sheets-Sheet 4 OUTPUTINVENTOR. ARTHUR F. COOPER ATTORNEY United States Patent 3,070,071FAELURE FRQTECTED SIGNAL TRANSLATING SYSTEM Arthur F. Cooper, Santa Ana,Calif., assignor to North American Aviation, Inc. Filed Mar. 11, 1960,Ser. No. 14,301 Claims. (Cl. 121--41) This invention relates to signaltranslating systems, and particularly concerns a multi-channel systemwhich pro vides protection against partial failure.

The problem of apparatus reliability and longevity becomes of greatersignificance with increasing complexity of modern equipment. Closed loopor feed back systems are commonly utilized links in the over lengtheningchain of components employed in electrical, mechanical,electro-hydraulic and electro-mechanical apparatus. A hydraulic servosystem, for example, may include a hydraulic motor having anelectrically-operated control valve and an error generating electricalsystem producing a driving error signal for operating the control valvein accordance with the difference between a commanded input and themotor output. In such a servo system, failure will occur most frequentlyin the electrical system or in the motor control system. Servo failuremay be such as to provide either zero or maximum output for any giveninput. The latter type of failure, which may be termed hard overfailure, is most troublesome since it prevents control by an alternateor manual system without totally disabling the failed system.Accordingly, it is an object of this invention to provide a system inwhich, while failure is a recognized possibility, the disadvantageouseffects of failure are minimized and increased reliability is achieved.

The principles of the present invention are applicable to a wide varietyof different types of signal translating systems whether either or bothinput and output signals are electrical, mechanical, fluid or the like.

In carrying out the principles of this invention in accordance with apreferred embodiment thereof, there is provided a signal translatingsystem embodying a plurality of substantially similar feedback or closedloop channels, each of which includes an output or power device which isoperable in response to a driving error signal. The driving error signalin each of one or more channels has combined therewith a portion of thedriving error signal in one or more of the other channels. The outputsof the individual channels are summed. That is, they are arranged toprovide a combined output drive from the system in accordance with thesum of the outputs of the individual channels.

In one specifically described embodiment, for example, the outputdevices of each channel are motors comprising valve-controlled fiuidmotors each arranged with a feedback loop to provide a position followup servo. All channels are responsive to a common command signal inputor to mutually equivalent command signal inputs and all outputs areadditively combined by suitable mechanical connections. Cross feed ofall error signals (which drive the respective motors) is provided bycombining with each error signal a portion of each of the other of theerror signals.

With the above-described arrangement, using solely two channels, thecombined mechanical output of the twochannel system will reflect verylittle or none of a hardover failure (a steady maximum output) of asingle channel. The system merely becomes inoperable whereby a monitorsystem may be used to de-energize the failed channel. If three or morechannels are utilized, upon failure of one channel the system willcontinue to provide linear operation without any monitor system.

An object of this invention therefore, is to increase the reliability ofa signal translating system.

Still another object of the invention is to minimize the disadvantagesof diiferent types of servo failure.

A further object of the invention is to provide a multichannel servosystem wherein the output will not reflect the failure of any singlechannel.

Another object of the invention is to provide a multichannel servosystem capable of operation after failure of one channel.

Still another object of this invention is the provision of amulti-channel servo system which will provide continuous operation inthe presence of failure of one or more of its channels.

These and other objects of the invention will become apparent from thefollowing description taken in connection with the accompanying drawingsin which:

FIG. 1 is an illustration of a two-channel servo system embodying theprinciples of this invention;

FIG. 2 illustrates certain details of a three-channel system;

FIG. 3 illustrates a mechanical-hydraulic system;

FIG. 4 illustrates a modified mechanical-hydraulic system, and

FIG. 5 illustrates a closed loop rate system.

in the drawings, like reference characters refer to like parts.

The term servo or servo system as utilized herein designates :aconventional closed loop or feedback system having an output produced inaccordance with an input and wherein a driving error signalrepresentative of the difference between input and output is applied todrive a signal translating or amplifying device within the system.

The system of the invention incorporates for each of its servo channels:a substantially conventional servo system including a signaltranslating device such as an amplifier or a motor, whether electrical,hydraulic, or pneumatic, and a feedback loop providing a conventionalclosed loop servo system. In accordance with certain embodiments of theinvention, two or more of such channels are interconnected by feedingthe error signal in each to the input of the others. The mechanicaloutputs of the motors are combined so that while each motor may operateindependently of the others (except for the cross feed) the systemoutput comprises displacement of a driven member in accordance with thesum of the mechanical outputs of the several motors.

As illustrated in FIG. 1, a single conventional electrohydraulic servochannel includes :an electrical-to-mechanical converter such as avalve-operated fluid motor 10. Motor 10 comprises a cylinder 12reciprocally mounting a piston 14 under control of anelectrically-operated valve 16 which supplies fluid under pressure toone side or the other of the piston 14 in accordance with the polarityof an electrical valve control signal appearing on lead 18. The valvecontrol signal on lead 18 is provided as the output of an amplifier 20which receives the driving error signal appearing on lead 22 at theoutput of a summing network 24. As in conventional servo operation, thesumming network 24 receives a command signal at input 26 and also anoutput position feed back signal on line 28 from a position pickofr 32.The pickofi 32 may comprise a potentiometer 33 having relatively movablearms 34 and 35 fixed respectively to the cylinder 12 and piston 14whereby the potentiometer pickofi provides an output voltageproportional to the relative displacement of piston and cylinder. Theerror signal output of the summing network 24 is, of course, thedifference between the inputs thereto on lines 26 and 28.

The above-described position follow-up servo channel comprises nothingthat is novel in itself and is considered to be exemplary of but one ofthe many types of similar closed loop servo systems well known to thoseskilled in the art. The second chanel of the two-channel systemillustrated in FIG. 1 is substantially identical to the first channeldescribed above and comprises a valve-operated motor 40, cylinder 42,piston 44, valve 46, amplifier 56, summing network 54, and pickofi 52,all constructed and arranged as are the corresponding elements of thefirst servo channel.

In order to provide fail-safe operation of the system of FIG. 1, the twochannels are interconnected by cross feeding the error signal in each tothe input of the other and by combining the outputs of the individualmotors. The error signal in the first channel which appears at theoutput of summing network 24 is fed through a network 60, having a gainK as a third input to the summing network 54 of the second servochannel. Similarly, the error signal appearing at the output of thesumming network 54- of the second servo channel is fed through a network61, having a gain K as a third input to the summing network 24 of thefirst servo channel. The gains K and K are preferably equal and lessthan unity, and must never be greater than unity.

A convenient, but exemplary arrangement, for adding the individualoutput displacements X and X of the respective motors 10 and 40 isillustrated as simply comprising a rigid interconnection of the twocylinders 12 and 42. With this arrangement, either piston such as thepiston 14 of motor 10 is connected to a base or support member 62 whilethe other, piston 44 of motor 40, will be secured to a member (notshown) which is to be driven by the system. Thus, the total output displacement X of the dual channel system relative to the support member 62comprises the sum of output displacements X +X While the inputs to thetwo channels are illustrated as being derived from a common inputterminal 64, it will be seen that individual equal or equivalent inputsmay also be provided.

It may be readily appreciated that any number of channels can becombined in the manner described above in connection with FIG. 1 withthe error in each channel being fed, all with equal gains, to everyother channel which is used. Further, all of the individual outputs X XX may be added in such an arrangement to give the total output X Such anarrangement is illustrated for three channels in FIG. 2 which alsoillustrates details of the summing networks and cross feeding of errorsignals. As illustrated in FIG. 2, the first channel may comprise thepreviously described fluid motor 16 with its valve 16 and pickoflf 32,amplifier 20, and a resistive summing network. Similarly, the secondchannel will comprise, a described in FIG. 1, the fluid motor 40 withits valve 46 and pickoff 52, amplifier 50 and a similar resistivesumming network. The third channel will be substantially identical tothe other two channels comprising a fluid motor 65 having a controlvalve 66 and pickoif 67, together with an amplifier 68 and a similarresistive summing network. The error signals in the respective channelsappear at the junctions 70, 71, and 72 of the several resistors of therespective summing networks to comprise the input to the respectiveamplifiers 29, 50, and 68. In the first channel (which operates motorthe common input signal is fed to one input resistor 72 of the summingnetwork while the feedback signal from pickoff 32 is fed to a secondresistor 73 of the summing network. The error signal in the secondchannel [from junction 71 is fed to resistor 74 of the first channelsumming network while the error signal in the third channel at junction72 is fed to resistor 75 of the first channel summing network.Similarly, each of the second and third channels has the common inputsignal fed to summing network resistors 76 and 77 respectively and itsindividual feedback signal from pickoifs 52 and 67 fed to summingnetwork resistors 78 and 79 respectively. In the second channel, summingnetwork resistors 80 and 81 receive the error signal from the first andthird channels at junctions 7t and '72 respectively. In the thirdchannel, the summing network resistors 32 and 83 receive the errorsignals from the junctions and 71 of the first and second channelsrespectively.

When these error cross feed networks are pure resistors, pairs ofresistors in parallel can be physically combined into one resistor, suchas 83 and 81, 74 and 80, and 82. However, separate resistors are shownfor ease of visual perception and understanding of the basic operations.

It will be seen that the error signal in each individual servo channelis summed with the error signal in each of the other servo channels withthe same cross feed gain between any two channels since all of resistors74, 75, 80, 8t, 82 and 83 are of equal value. The cross feed resistorsare so chosen that the cross feed gain is less than one-half for thisthree-channel system. That is, the magnitude of the cross-fed signalcomponent appearing at any one summing network output is less than themagnitude of the error signal from which such component is derived. Thegain may be more rigorously defined by:

where K =cross feed gain, nznumber of channels. The expression on theleft side of the equation must always be positive.

it may be noted that gain (or resistor) matching in the error networksis not critical. Mismatching, alone, will not cause instability.

As in the two-channel system, a system involving three or more channelswill have the outputs of the individual channels combined. Thus, asillustrated in FIG. 2, the outputs of motors it) and 40 are combinedjust as described in FIG. 1 while the output of motor 65 is combinedwith the other two motor outputs by having the cylinder thereof securedto the piston of motor 40. Thus, it will be seen that the motors areconnected in tandem each motor having first and second parts, such asits cylinder and piston, mounted for relative motion in response to theerror signal input which drives the motor valve. Each motor has one ofits parts, such as the cylinder part of motors 1t) and 46, fixedlyconnected to one part of the next adjacent motor while having the otherof its parts, such as the piston 14, mounted for motion relative to theother part such as piston 44 of the next adjacent motor 40.

It will be seen that in the embodiments described, as in others to bedescribed hereinafter, the several channel outputs comprise mechanicalmotions that are mutually independent. For example, the output of motor10 is the motion of cylinder part 12 relative to piston part 14. Thismotion is in no way affected (in the absence of cross feed of errorsignals) by the output of motor 40 which is the motion of cylinder part42 relative to piston part 44. The several mutually independentoutputmotions are combined as illustrated. With this arrangement, forexample, one channel may be locked without preventing operation ofanother.

In normal operation (FIG. 1) the error signal in each channel crossfeeds into the other channel and thus enhances and supports theindividual outputs X and X of the respective channels. If one channel,such as that providing the output X fails and goes hardover due to suchfailure, its error becomes large and feeds into another channel such asthe channel providing the output X In this'situation, the second channeloperates in response to a total error signal having as one componentthereof its own normal operation error signal and having as a secondcomponent thereof a portion of the error signal of the failed channel.In such a situation the output X of the operating channel becomes ofopposite sense to the output X of the failed channel. Since the systemoutput X is the summation of the output X and X the total output remainszero or at a relative small value depending upon the magnitude of thefractional cross feed gain. This cross feed gain, while being less thanunity, may have any desired value such as, for example, 0.90-0.98.Preferably, the cross feed gain is as close to unity as possible. Thesystem is unstable for a gain greater than unity. Due to inherent lackof absolute precision of the gain controlling components and to insurestability, the gain in this system is reduced.

In the above-described operation of a two-channel system with a hardoverfailure, the system output does not become maximum but does, in effect,provide a fail-safe condition. Nevertheless, the system is no longeroperable. However, it will be readily appreciated that the failure ofany single channel may be sensed by any suitable monitor system wellknown to those skilled in the art which may be utilized to shut off thepower to the failed channel and recenter the latter to zero. Thus, theremaining channel could still be utilized to provide the total output.

The cross feed system lends itself to a very simple monitor system for ahardover failure (the most common type). A large pressure differentialexists across the piston of the bad channel, while the good channelshave zero pressure differential. Pressure switches with suitable timedelay can positively sense the bad channel. Auxiliary devices can cutout the bad channel and allow the good channels to continue operation.

This monitor sensing capability is one of the appeals of using the crossfeed system compared to other multichannel schemes, particularly for adual channel scheme Where continued operation after a hardover typefailure is desired.

It will be noted, however, that if more than two channels are used andcombined as illustrated in FIG. 2, for example, the system will continueto operate linearly without any monitoring system even after failure ofone channel.

A steady state operation under normal and failure conditions for bothtwo and more than two (11) channel systems is shown in the table belowin terms of expressions relating the individual outputs X and X and thetotal output X to the input R. Operation under two types of failures isdescribed. The first type of failure is defined as a zero type whereinany one individual output such as X goes to zero and remains there forany input R. A second failure, the hardover type, is defined as afailure such that any one individual output such as X; goes to itsmaximum limit value and remains there for any input R.

channel system where n=2, the cross feed error gain K must be less than1.

From the above table, it will be seen that zero failure of one channelof either a dual or n system will not prevent continued linearoperation. Hardover failure in a two-channel system provides a fail-safecondition wherein the outputs X remains zero or a small value, dependingupon the gain K With hardover failure of a single channel of an nchannel system, only a small fraction of the failed channel outputappears in the total system output X whereby the system will continue tooperate linearly with its two or more non-failed channels. For example,with reference to FIG. 2 if hardover failure of the channel includingthe motor 65 occurs such that the piston of this motor is moved to theextreme left in the drawing, the cross feed from the failed channel tothe other two channels will operate to cause the piston of motor $0 tomove toward the right relative to its cylinder and the cylinder of motor10 to move toward the right relative to its piston an amount sufficientto compensate for the failed position of the motor 65. The latter maythus remain in its failed limit position while motors 10 and continue tooperate linearly in response to the system input.

In the triple channel system of FIG. 2, the desired output of the systemcomprises displacement of a member (not shown) which will be secured tothe piston of motor 65 relative to a fixed support 62 which is securedto the piston of motor 10. It is to be understood that any suitablemeans may be provided to constrain relative motion of the severalmotors. In the disclosure of the relative motion of the individual motorparts such constraint is schematically illustrated in FIG. 2 as rollers90 mounted between the several motors and the base or support 62.Additional rollers or slidable constraint may be provided if deemednecessary or desirable.

Illustrated in FIG. 3 is a mechanical-hydraulic system having adifferent type of summing of outputs, a mechanical signal input, and amechanical linkage arrangement for the error network and cross-feed.There is provided a mechanically operated valve assembly 101 which isarranged to feed high pressure oil to or from actuator 104 through ports102 and 103. The ram or piston 105 of the actuator moves left or right,depending on which side of the ram is supplied with high pressure. Asecond identical system of valve 101a and actuator 104a is provided forthe dual system illustrated. Ram 105 is connected to a mechanical summeror walk- Normal Zero Failure Hardover Failure X /R Xj/R Dual system 28 Sn Channel sys- 118 S tem.

In the above table,

It will be seen that for a multiple channel system of any number (n) ofchannels, the cross feed gain between any two channels (equal for allchannels) must be such that (n1)K is less than unity. Therefore, in adual ing beam 106 at pivot point 107, while the piston a is connected towalking beam 106 at pivot point 107a. The motions of the rams 105 and105a are summed through the beam 106 to produce the net desired outputin the form of motion of an output member 108 pivoted to the beam 106 atpoint 108a.

The valve 101 is operated by input motion left or right supplied throughlink 109 which is pivoted to a valve clapper arm 110. It is to be notedthat this description of the operation of valve 101 is equallyapplicable to valve 101:: and its actuator 104a. Parts in the secondchannel of the dual servo system are designated by numbers correspondingto like parts of the first channel with the ad dition of the suffix a.

Clapper arm 110, pivoted in the body of valve 101, is operated by inputlink 109 to close either of orifices 111 or 112 formed in the valvebody. High pressure from a pressure source, not shown, is supplied tochambers 113 and 114 and via pressure dropping orifices 115 and 116,

flows to return chamber 11"] through one of orifices 111 and 112 whichis not closed by the clapper arm. A valve spool 118 is mounted in thevalve body and normally centered by springs 119 and 120. Upon closing ofone of orifices 111 or 112, there is created a differential pressureacross the valve spool in chamber 121 and 122 provided between therespective ends of the spool and the valve body. This differentialpressure moves the valve spool to the left, for example to a point wherecommunica tion between pressure chamber 113 and conduit 102 is providedtogether with connection between chamber 117 and conduit 103. in suchposition the ram 105 is moved toward the right in the illustration aslong as the pressure differential is maintained across the valve spool.Upon movement of the valve spool to the right by creation of an oppositesense pressure differential, conduit 103 is in connection with highpressure chamber 114, which conduit 102 connects with return chamber117.

To summarize the operation of the valve actuator com" bination for onedirection of motion, arm 109 moves to the right, causing clapper 110 toclose orifice 111. and provide a larger pressure in chamber 121 than inchamber 122. Spool 118 therefore moves to the right provid ing highpressure from chamber 114 through conduit 103. A return path is nowprovided from conduit 102 to chamber 117. Ram 105 therefore moves to theleft and continues to so move until input arm 109 is returned to neutralposition by the error forming linkage to be subsequently described. Whenarm 109 is returned to neutral, the pressure in chambers 121 and 122 isequalized and the springs return the spool 118 to neutral whereby theram 105 stops moving.

The mechanical linkage illustrated between the two valves, providescross feed of error to each valve input and also provides the positionfeedback which completes the closed loop of each servo system. Thecommon input signal to each servo system is a mechanical signal in theform of motion to the left or to the right of an input link 123. Inputmotion is transmitted through link 123 to a lever 124 which pivots aboutpoint 125. Point 125 is held fast by connecting arm 126 which is rigidlyattached to the ram 105. Pivotal motion of link 124 about point 125 istransmitted through a pivotal connection 127 to a link 128 pivoted at129 to one end of a link 130. The input arm 109 is pivoted at 131 to anintermediate portion of link 130.

The input motion of beam 123 is applied to the clapper driving arm 109aof valve 101a via similar linkage including links 124a, 128a, and 130a.Accordingly, upon motion of input beam beams 124 and 124a pivot aboutpoints 125 and 125a to move links 128 and 128a to the right. There isprovided a link 133 rigidly affixed to link 128 and pivoted to link 130aat point 1320. There is provided a link 134 rigidly affixed to link 128aand pivoted to link 130 at point 132. Thus, upon motion of beams 128 and128a to the right, beams 130 and 130a move to the right without pivotingsince both links 133 and 134 move equally to the right. I

Upon movement of input arm 123 to the right, the above described linkagemoves both arms 109 and 109a to the right in equal amounts and operatesthe valve actuator system as previously described. Accordingly, bothrams 105 and 105a move to the left equal amounts producing the desiredoutput motion of arm 108 which is equal to one-half the sum of themotion of the two rams.

Position feed back results when motion of the ram such as 105 istransmitted by link 126 to point 125 moving this point to the left. Thiscauses arm 124 to pivot about its pivotal connection to the input beam123 to move arm 128 to the left and return arm 109 to neutral wherebythe motion of the ram is stopped. Similarly, link 126a provides positionfeedback for the second channel.

The error signal in each servo system is, in this embodiment, the motionof the respective beams 128 and 12811.

123 to the right, for example, both The error motion of beam 128 iscross fed to the other channel by link 133 while the error motion ofbeam 128a is cross fed by means of link 134.

Under failure condition, it is assumed that, for example, a metal chipis lodged between the valve spool and the valve body such that actuator104 is continuously pressurized to move ram hardover to the left whereit remains regardless of input motion of beam 109. Motion of ram 105 tothe left is transmitted by link 126 to move pivot point to the leftthereby pivoting arm 124 about input beam 123. Motion of arm 128, theerror, is to the left and via cross feed link 133 causes arm a to pivotto the left about pivot point 129a. This causes arm 109:! to move to theleft and effect motion of ram 105a to the right. Ram 105a moves to theright substantially as far as ram 105 moves to the left and no netoutput motion of arm 108 results. Right motion of ram 105a istransmitted via 126a to link 124a which pivots about its connection tolink 123. This moves link 128a to the right to bring link 109a toneutral position. Link 109 also returns to neutral by virtue of thecross feed motion through link 134 but under the asumed failureconditions, link 109 has no effect upon ram 105.

It is understood that little force is required to operate valve arms 110and 110a while input beam has a substantial source impedance.

The cross feed gain is determined by the relative lengths of the twoportions of links 130 and 130a, respectively. Thus, to preventre-generation and provide a gain of less than unity, the top section ofthe link between pivot points 129 and 131'is made slightly shorter thanthe bottom section of the link between pivot points 131 and 132. Withthe cross feed gain of less than unity, the net output would not beexactly zero under the described failure conditions.

The system of FIG. 3 is a direct corollary to the previously describedelectrical cross feed error system wherein the error of any one systemis continually fed to the other system. Two separate actuators areutilized and the outputs added through a Walking beam arrangement whichis an alternate way of mechanically combining outputs of hydraulicactuators.

Illustrated in FIG. 4 is a somewhat different embodiment which employscross feed of error in a manner somewhat different than that shown inthe other systems. Further, the operation of the system of FIG. 4 duringfailure differs in that the output motion can reflect directly throughthe valve motor combinations to the input.

The system of FIG. 4 includes a pair of hydraulic actuators and 140aconnected to a fixed support structure 143. The actuator cylinders areconnected at pivot points 144 and 144a to the respective ends of asummer or walking beam 145 having an output shaft 146 pivotallyconnected at 147 to an intermediate point thereof. Valves 148 and 148aare rigidly affixed to the respective actuators to move therewith. Thevalves include valve spools 149 and 149a which are directly operated bymechanical motion of input arms 150 and 150a respectively. The inputarms are connected in turn to opposite ends of a beam 151 at pivotpoints 152 and 1520 respectively. The common input is provided by motionto the right or to the left of an input shaft 153 which is pivotallyconnected to the mid-point of beam 151 at point 154.

If input beam 153 is moved to the right, the motion is transmitted tothe two servo channels and moves arm 150 and 150a to the right. Thevalve spools 149 and 149a move the the right equally relative to thevalve housings. High pressure oil which is provided at all times from asource (not shown) to valve ports 155, 156, 155a and 156a is transmittedthrough ports 155 and 155a to actuator chambers 157 and 157a. Also uponmotion of the valve spools to the right, valve chambers 158 and 158a areconnected to the valve port's 159 and 1590 which are at all timesconnected to relatively low pressure or returns. Accordingly, theactuator cylinders move to the right (relative to common support 143)until the valve spools 9 are again returned to neutral (illustrated)wherein the ports 155, 156, 159, 155a, 156a and 159a are blocked. Theapparatus is arranged so that the force required to move the valvespools is substantially less than that required to move input beam 153.Therefore, upon motion of the actuator cylinders, the valve bodies whichare fixed thereto are moved relative to the valve spools. Thisconstitutes a position follow-up. That is, output motion of anindividual actuator results in an input motion of its own valve spool ina sense and amount sufiicient to null the input to the individual servochannel. The spools now having returned to neutral, the actuators arefixed in position. The net motion of actuator cylinders 144i and 140a tothe right is summed in beam 145 and produces the net output in arm 146.

Assuming a failure condition exists such as, for example, the lodging ofa chip between a valve spool 149 and its housing 148, chamber 157 ofactuator 140 is subjected to continuous high pressure while chamber 153is subjected to continuous low pressure. Actuator 140 moves to the rightuntil a limit position is reached. The chip of the assumed failurecondition prevents closing of ports 155 and 159 by the normal follow-upaction. That is, rela tive motion of valve spool and body cannot occurbecause of the chip. Therefore, as the actuator cylinder 1.4% is movedto the right, the valve spool 149 together with its operating arm 150,is dragged to the right. Since no input exists in this assumed failure,the second channel of the dual system (that including the actuator 14%)is still at neutral. A torque is applied to beam 151 by virtue of themotion of arm 150 and the resistance to motion of the input arm 153.Therefore, beam 151 rotates about pivot point 154 (which has somethreshold friction level) whereby the upper end of beam 151 moves to theright and the lower end of the beam moves to the left, both in equalamounts. Arm 150a and its spool 149a are moved to the left causingactuator 140a to move to the left an amount equal to the motion ofactuator Mil to the right. The motion of the two actuators, one to theright and the other to the left, is summed in beam 145 whereby the netmotion of the output arm 146 is zero in this condition of hard-overfailure of the upper channel of the system.

The failure of actuator 140 is reflected in motion of arm 150 of itsvalve. This is effectively an error which is cross fed to the secondsystem by means of rotation of beam 151 about pivot point 154 whichcreates a resultant motion of arm 150a of the second channel.

Illustrated in FIG. is an example of the application of the principlesof the present invention to an electromechanical dual rate orintegrating servo system (as distinguished from previously describedposition systems) 1f] the motor 169. Thus, the output X is the timeintegral of the input on lead 165. That is, for a constant input, theoutput X increases at a constant rate. Also, the rate of change of theoutput voltage is proportional to the input.

There is provided a second identical rate servo also driven from thecommon input 165 and providing an output X This second channel includesthe error summing network 166a, amplifier 167a, motor 169a, tachometergenerator 172a, gearing 171a, and pickoff potentiometer 175M, allillustrated and arranged as are the corresponding elements of the firstchannel of this dual rate system.

The outputs X and X of the two servo channels are electrically combinedin summing network 174 to produce at output terminal 175 the totalsystem output X The driving error signal of the several channels appearsat summing points 176 and 176a of the respective summing networks. Eachdriving error signal is fed to the summing network of the other channelvia the common resistor 177 which provides the desired cross feed gain.

Operation of the system under failure condition is similar to theelectro-hydraulic position systems of FIGS. 1 and 2. Three types offailures are protected against in the system of FIG. 5.

(1) Either of the individual outputs X or X is zero and the pickoffpotentiometer associated therewith does not move.

(2) Either X or X is constant but does not change with time.

(3) Either X or X runs away at a constant rate.

It is assumed that the tachometer generator is operative as long as itsassociated motor is operative. For the first menlioned failure, thenon-failed system picks up the entire load and the output isapproximately the same as it would be in the absence of failure,depending upon the value of the cross feed gain. It is noted that inthis rate system, it is preferable that the cross feed gain be madeequal to unity and that in this situation, the gain can be greater thanunity without instability.

For the second type of failure, the non-failed system picks up theentire load, but now the output has a con stant (position) bias due tothe constant output of the failed channel. Nevertheless, the outputrate, the rate of change of X is still proportional to the input.

For the third type of failure wherein the failed system runs at aconstant rate, the second system by virtue of the cross feeding of error(which in this case represents rate) is caused to run at a rate equaland opposite to the failure rate whereby the net output X is zero.

The following table is descriptive of the rate system of of FIG. 5.

employing electric cross feed of error for failure protection. Thecommon input signal is fed from input terminal 165 to the summing anderror forming network 166 of the first channel servo. The first channelservo includes ;put signal at lead 165. The output voltage of thepotentiometer 170 is the individual channel output X which isproportional to the total angular shaft displacement of K-Gain ofamplifier and motor combination.

K Gain of tachometer.

K Cross feed error gain.

S-Scale factor depending on gear-train and input and output and potscaling.

xf-A constant rate of x output.

R-Input.

In the system of FIG. 5, there are no critical parameters. Parameters ineach channel should however be of substantially equal value. The crossfeed gain does not have to be less than unity as in the position systemand shoud be as close to unity as possible.

There have been described a number of different mechanizations of amulti-channel servo system which is protected against failure withoutnecessity of external dividual thereto,

monitoring apparatus. While the systems of FIGS. 1, 3, 4 and 5 have beendescribed in connection with two channel systems, it will be readilyappreciated that each of these systems can be mechanized utilizing threeor more channels, combining all outputs and cross feeding all drivingerror signals substantially as described in the embodiment of FIG. 2.Each of the described systems employs an improved structure and methodfor operating a number of closed loop signal translating systems from acommon input by virtue of combining with the error signal of one servoloop at least a portion of the error signal of another of the servoloops and further combining the outputs of the servo loops.

It will be seen that the above-described interconnection of conventionalservo channels provides a highly reliable servo system wherein all ofthe components are utilized for normal operation. Failure of one channelof a dual channel system will have little effect on the system outputwhile operation of a three or more channel system will be continuousdespite failure of any one. Neither gains nor balance are criticalproviding only that the cross feeds be limited as described above.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the sarre is by way of illustration andexample only and is not to be taken by way of limitation, the spirit andscope of this invention being limited only by the terms of the appendedclaims.

1 claim:

1. A multi-channel servo system comprising first and secondelectro-mechanical converters each providing a mechanical output inresponse to an individual electrical error signal input, means foradding said converter outputs, first and second pickofif means forgenerating first and second pickoft signals in accordance withrespective converter outputs, first and second summing means eachindividual to a respective converter for algebraically combining acommon input control signal with respective pickoff signals to producesaid individual error signals, and cross-feed means for modifying atleast one of said error signals in accordance with the other.

2. A multi-channel servo system comprising first and secondelectro-mechanical converters each providing a mechanical output inresponse to an individual electrical error signal input, means formechanically connecting "said converters to provide a combined systemoutput in accordance with the sum of the individual converter outputs,first and second pickoif means for generating first and second pickoiisignals in accordance with respective converter outputs, first andsecond summing means each individual to a respective converter foralgebraically combining a common input control signal with respectivepickoif signals to produce said individual error signals, and cross-feedmeans for adding to at least one of said error signals a portion of theother.

3. A multi-channel servo system comprising first and secondelectro-mechanical converters each providing a -mechanical motion outputin response to an individual electrical error signal input, eachconverter comprising first and second parts mounted for relative motion,each converter having one of its parts fixed relative to one part of theother converter and having the other of its parts mounted for motionrelative to the other part of the other converter whereby saidmechanical outputs are summed, first and second pickoti means eachindividual to a respective converter for generating first and secondpickoff signalsv respectively in accordance with relative motion of theparts of the converters infirst and second summing means each individualto a respective converter for algebraically combining a common inputcontrol signal with respective pickofi signals to produce saidindividual error signals, and cross-feed means for modifying at leastone of said .error signals in accordance with the other.

4. A servo system comprisnig: a plurality of servo 1) (-l channels eachindividually responsive to a command signal and each producing anindividual output as a function of said signal, each channel comprisingan electrical to-mechanical converter for producing a mechanical outputin response to an electrical error signal input thereto,- and means forproducing said error signal in accordance with the difference betweensaid command signal and the individual channel converter output;cross-feed means for combining with the error signal of each channel aportion of the error signal of each other channel; and means forcombining said individual outputs.

5. A servo system comprising: a plurality of servo channels allresponsive to a common command signal and each producing an individualmechanical output as a function of said signal; each channel comprisinga valve controlled fluid motor having piston and cylinder partsrelatively movable in response to an electrical error signal, pickotfmeans producing a pickoff signal in accordance with relative motion ofsaid parts, and means for producing said error signal in accordance withthe difference between said command and pickoit signals; cross-feedmeans for adding to the error signal of each channel a portion of theerror signal of each other channel; said motors being arranged in aseries, each motor having one of its parts fixed to one part of a nextadjacent motor and having the other of its parts displaceable withrespect to the other part of such next adjacent motor whereby relativemotion of parts of each motor comprises said individual mechanicaloutput and all such outputs are serially added.

6. A multichannel signal translation system comprising a plurality ofclosed loop signal translation channels each including an output devicefor producing an output in response to a driving error signal appliedthereto, means for combining with the error signal in each of saidchannels at least a portion of the error signal in another of saidchannels, and means for combining outputs of said channels.

7. A rnulti-channel actuator system comprising first and second valvecontrolled fiuid motors, each comprising a movable cylinder, a pistonmounted in said cylinder and connected to a support, a valve fixed tosaid cylinder for controlling fluid flow to and from said cylinder, saidvalve including a slidable valve core having a core driving linkconnected therewith; a summing link pivotally connected to both saidcylinders, said summing link being mounted for translational motion toprovide a combined motor output by translational motion of anintermediate portion thereof; an output link pivoted to saidintermediate portion of said summing link; a driving link pivo-tallyconnected to the core driving link of said first and second motors; andan input pivoted to an intermediate portion of said driving 8. Amulti-channel servo system comprising a plurality of motors connected intandem, each motor comprising first and second parts mounted forrelative motion in response to an error signal input thereto, each motorhaving one of its parts fixedly connected with one part of the nextadjacent motor and having the other of its parts mounted for motionindependent of the other part of said next adjacent motor, and means forcombining with the error signal of each motor a portion of the errorsignal of each other motor.

9. The method of operating a number of closed loop signal translatingsystems from a common input, each said system being a closed loop andhaving a driving error signal representative of the difference betweensystem input and output, comprising the steps of combining with theerror signal of a first of said systems at least a portion of the errorsignal of a second of said systems, combining with the error signal ofeach of said systems at least a portion of the error signal of the otherof said systems and combining the outputs of said first and secondsystems.

10. Multi-channel signal translating apparatus comprising a plurality ofclosed loop signal translating channels responsive to a common inputsignal, each channel comprising a signal translating device forproviding an individual channel output in accordance with a drivingerror signal input rthereto, and a summing device for producing saiddriving error signal as the difference between said common input andsaid channel output; crossfeed means for combining the driving errorsignal of each of said channels with the driving error signal of theother of said channels; and means for combining the individual channeloutputs of said first and second channels.

11. The apparatus of claim 10 wherein said signal translating device ofeach channel comprises an amplifier responsive to the driving errorsignal, an electric motor connected to be driven by said amplifier, anda tachometer generator having an input from said motor and having anoutput to said summing device.

12. The apparatus of claim 10 wherein said signal translating device ofeach channel comprises an amplifier responsive .to the driving errorsignal, and valve operated fluid motor connected to be driven by saidamplifier.

13. The apparatus of claim 10 wherein said input and error signalscomprise mechanical motions, said signal translating device of eachchannel comprising a fluid motor having a control valve and a valveoperating link connected thereto, said summing device comprising asumming linkage having an input link for receiving an input motion,having a feedback connection with said motor, and having a driving errorconnection with 14 said valve operating link, said cross-feed meanscomprising first and second cross-feed links interconnecting the summinglinkage of first and second channels.

14. A multi-channel servo system comprising a plurality of closed loopservo channels each transmitting an error signal to produce a mechanicaloutput displacement in response to an input, means for combining withthe error signal in each of said channels a portion of the error signalin another of said channels, and means for combining mechanical outputdisplacements of said channels.

15. A multi-channel signal translation system comprising a first closedloop signal rtranslation channel having an output device for producingan output in re sponse to a first driving error signal applied thereto,a second closed loop signal channel having an output de vice forproducing an output independent of said first named ouput in response toa second driving error signal applied thereto, said channels includingmutually independent feedback means, means for effecting cross feed ofsaid error signals between said closed loop channels, and means forcombining outputs of said channels.

References Cited in the file of this patent UNITED STATES PATENTS1,353,656 Heisler Sept. 21, 1920 2,561,654 Eller July 24, 1951 2,597,361Mott May 20, 1952 2,763,990 Mercier Sept. 25, 1956 2,856,947 Hart Oct.21, 1958

